A nimbus is a glowing light that encircles someone or something. A shadowy figure standing in front of your car's headlights might have a nimbus around it.
One kind of nimbus is the holy glow that's said to surround the body or head of a saint — a halo, in other words. Many religious works of art feature Christian saints, each with a radiant nimbus. There is a more scientific kind of nimbus as well, a large, gray rain cloud. In Latin, nimbus simply means "cloud," possibly connected to nebula, "mist, vapor, or fog."